Mesoporous silica is a new material having pores with a diameter of 2 to 50 nm (to be referred to as “mesopores” hereinafter) and expected to be used in various fields such as catalyst support and separating agents. Like other inorganic materials, it is preferably in the form of fine particles in most cases when it is actually used.
For example, when it is used as thin film like an ink absorbent for ink jet recording paper or low-dielectric thin film, to obtain a flat homogeneous film, mesoporous silica must be particulate and submicron-sized mesoporous silica particles are needed.
In the fields of catalyst support, separating agents, adsorbents and medical carriers for medicines, mesoporous silica is granulated, molded or dispersed uniformly in a matrix. To improve the mechanical strength of a granulated or molded product or dispersibility in the matrix, mesoporous silica must be particulate.
Under the above situation, mesoporous silica must be particulate. However when mesoporous silica is pulverized into fine particles, mesopores that are the greatest feature of mesoporous silica collapse, resulting in a greatly reduced value as a material.
Particularly when mesoporous silica is pulverized into submicron-sized fine particles, the collapse of mesopores is marked and the volume of mesopores in mesoporous silica greatly decreases.
In view of the above problems, the inventors of the present invention propose a process for obtaining particulate mesoporous silica by processing a mixed solution of mesoporous silica and a cationic resin dissolved in an aqueous solvent with a high-pressure homogenizer (JP-A 2002-356621) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
However, to make mesoporous silica fully particulate by the above process, the mixed solution must be processed with the high-pressure homogenizer many times in most cases, leaving room for the improvement of production efficiency. Since mesoporous silica produced by the above process contains the cationic resin for the prevention of the collapse of its mesoporous structure, its application is limited and there is a problem with uniformity in mesopore size due to a wide mesopore distribution. Therefore, it is difficult to use it as a catalyst support or separating agent which is used for a specific-sized substance.
As means of pulverizing mesoporous silica while the collapse of its mesoporous structure is prevented, wet pulverizing using an organic solvent as a dispersion medium is proposed (JP-A 2000-44227).
Although the above method provides a certain measure of effect, in order to prevent the collapse of mesopores, mesoporous silica can be pulverized into particles of a size only about 10 μm. When it is pulverized into submicron-sized fine particles, the volume of mesopores in mesoporous silica greatly decreases.